UCS and CCS are important criteria in the geological assessment of subterranean formations (rock), especially in the extraction of subterranean oil, gas, water and other minerals. One way of several in which UCS and CCS measurements are used in these industries is to determine the extent to which a particular formation can be drilled before it becomes necessary to replace part or all of a drill bit used for this purpose. The fundamental selection of drill bit type (e.g. in terms of surface material, geometry and other parameters) can also be assisted through knowledge of UCS and CCS data.
Broadly stated, UCS is a measure of the strength of rocks and cohesive soils under no confining pressure, and for reasons such as those given above is an important parameter in the mining, drilling, water extraction, oil and gas industries and to which the invention pertains in particular.
UCS traditionally was measured using a laboratory test at atmospheric pressure based on e.g. core samples. More recently dynamic logging measurements have been used to calculate UCS. This avoids the problem, that may be extant in e.g. a borehole in an oil or gas field, of recovering a rock sample to a surface location for testing.
UCS is known generally to understate the true compressive strength of rock in any event, because it cannot take account of the known strengthening effect of confinement of subterranean rock caused by overburden and a number of other geological factors. Partly for this reason CCS is usually seen as a more realistic measure of rock hardness in drilling and well logging operations.
Conventionally in the logging of oil and gas fields a single expression is used to establish UCS once log data such as sonic log data are obtained. It is known that numerous expressions potentially are available for calculating UCS but the use of a single expression seems to reflect the significant perceived difficulty in allocating expressions correctly in a field situation, in which rapid and reliable UCS and CCS calculation may be required. The most commonly used expression format however does not take account of variations in lithology, location, available input variables and data quality.
The invention seeks to improve the accuracy with which UCS and CCS values may be estimated or calculated at high speed, as may be required for instance during a logging-while-drilling (LWD) operation or before a drilling or other subsurface operation commences. At such times a drilling engineer may require rapidly updated rock strength information such as UCS or more likely CCS data, such that he/she can make instantaneous decisions about variables such as drilling rates of penetration, drill rotation speeds, drilling directions, drillhead angles/orientations, drillhead cutter selections, cutter materials or specifications and drill wear gradings (related to drill service lives) in prevailing conditions. Accurate UCS and CCS data can additionally be used to assist in selection decisions relating to downhole equipment other than drill tools, and wellbore stability issues